Light projector



F. A. BENFQRb 'LIGHT rnomcron Filed Nov. 30. 1925 4 Sheets-Sheet 1 GENE/PATIVG ANGLE d- 056/?![5 lnventot M/ Q F. A. BENFORD Aug. 27, 1929.

LIGHT PROJECTOR Filed Nov. 50. 1925- 4 Sheets-Sheet -2 Fig.7.

'0 5 /o 5 20 2530 as 4a 4550 Z (DEGREES) Invehton:

M a W Aug. 27, 1929. F. A. BENFORD 6,379

LIGHT PROJECTOR I File d Nov. 30. 1925 4 Sheets-Sheet s Fig. 9.

Inventor I Aug, 27, 1929. F. A. BENFoRo 1,726,379

- LIGHT PROJECTOR Filed Nov. so. 1925 4 Sheets-Sheet 4 amt-04mm cu/rvz I Inventor Z Patented Aug. 27, 192-9.

- UNITED STATES PATENT OFFICE.

FRANK A. BENFORD, OF SCHENECTADY, NEW YORK, ASSIGNOB TO GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK;

LIGHT PROJECTOR.

Application filed November 30, 1925. Serial No. 72,127.

This invention relates to light projectors having a reflector made up of a number of small mirrors or reflecting surfaces. The object of the invention is to provide an improved arrangement and proportioning of the elements of such a sectional reflecting surface; to provide a reflector in which the individual mirrors or reflecting surfaces are arranged in such a way with respect to or about a focalizing surface such as a paraboloid that the composite beam resulting from the merging of a multitude of individual beams will be free from images, and highly uniform in intensity.

More particularly the object of the inventionis to provide a reflector made up of a plurality of flat reflecting surfaces or sections adjusted to be in internal contact with a conoid (i. e. ellipsoid, paraboloid or hyperboloid) the length and width of said sections being so chosen that the divergence of the rays is substantially equal for all sections.

The invention will be more fully understood from the following specification in which the invention is more fully set forth and claimed, reference being had to the accompanying drawings in which Fig. 1 shows curves giving the relation between angular width of light source and the generating angle for use in determining the sections of a sectional mirror, the. curve marked Major axis being intended for use with a spherical source and the one marked Minor axis being intended for use with a disk-shaped source; Fig. 2 is a front view of one form of the projector of the invention showing the sections of the reflector; Fig. 3 is a section of a reflector in which the mirror sections are shown in tangential relation to a conicoid. In this figure open joints are seen which is not a desirable form of construction. Fig. 4 is a side view of a modified form of the invention; Fig. 5is a front view of the same device indicating the image formation of the light source in each section; Fig. 6 shows a conicoid designed to pass through the meridian center of the mirror sections. In this particular instance the conicoid is a parabothe angular length of the transverse axis of any mirror section which angular length w=p sine a;

F 9 is a sectional View through line 9-9; Fig. 10 1s a drawing illustrating how the angles are added; Fig. 11 shows a front view of a modified form of the projector of my invention; Fig. 12 shows a section taken along line 1212, Fig. 11; Fig. 13 is a front view of a sectional mirror; and Fig. 14 is a section on line 1tl4, Fig. .13, showing the focal point'F of the original conicoid and showing also F the focal point of the conicoid passing through the ends of the meridian central lines.

Nature of the beam from, a sectional mirror.

The striking features of a beam from a sectional mirror are its softness of outline, uniformity of intensity, and relative freedom from images of the source. These features are often of extreme value, but they are attained at the expense of, beam strength, so that to the above features might, be added that of low intensity and a wide angle of spread. The projection efficiency is ordinarily high, therefore for short range projection the floodlight built on the sectional plan stands without a superior. The beam formation takes place in a manner radially different from the parabolic and hyperbolic beams and as the analysis of the beam in-' volves new points of view, it; is necessary to begin a survey of the subject with the very elements of reflection by a'plane mirror, and by some process of summation arrive at the beam from the complete reflector.

The various beams of the conic family have one point in common :-they come from a single real or virtual image. The sectional beam however radiates from a multitude of images and the whole beam therefore is composed of a multitude of individual beams that, at a sufficient distance merge into one.

'T he u/m't beam.

- Each flat section of mirror in the sectional mirror reflects an independent beam of light I =Kl candles (357) If the centers of the elemental beams are substantially parallel then at some distance the will all overlap and, addin the direct ra iation of the lamp, the com ined beam strength is I=2Kl +l candles (358) Thus with a source giving 1000 candles uniror-of glass having a coeflicient of 0.85, the beam strength is I= 36 x 0.85 x 1000+ 1000=31,e00 candles of which 30,600 is the strength of the reflected light and 1000 is direct radiation from the source. Fig. 5 is a drawing of such a sectional mirror with the light source in the center and the 36 images in the 36 sections arranged in three concentric circles. In this actual case the intensity of radiation is not equal in all directions about the source, and the direct radiation along the axis of the beam is far below the average, as is clearly indicated in the figure.- In computing this beam each zone of mirrors must be used in connection with the correct intensity as shown by the distribution curve of the lamp, and for this reason the summation indicated in Equation (358) is more than a simple counting of sections.

Gondz'tz'ons for maximum beam zmiformity.

(1) Sphem'cal source-A condition that might be regarded as of prime importance is the attainment of a high degree of uniformity at all points in the strength of the beam. The beam from a paraboloid ordinarily has a center of nearly uniform intensity, the crest,

1 pressed as follows: formly in all directions, and a 36-section mlrest degree of beam uniformity. For practical purposes the computations of the angular dimensions of the sections of asectional mirror need not be carried out with exactness. Approximations are suificient. This condition allows a number of simplifications to be made in the formulas, all of which do not follow orthodox lines. An attempt to follow a rigid mathematical treatment soon leads to such complicated forms as to discourage the most conscientious designer, andtherefore the simple forms here given ma be of far greater practical use. Obvious y, the width W of the beam from a single mirror measured-in a meridian plane, is equal to the angular length, l, of the mirror, plus the angular width 8 of the source as measured from the center of the section. This may be ex W.=z+8 degrees 359 grees from the axis of the reflector, the equation becqmes+ In Fig. 10 the angle subtended by the source and indicated by in the above equation is abbreviated to e, and in this fi ure the width that has been added to the re ected beam by the size of the source is indicated by the dotted lines drawn tangent to the image and by the symbol W If the edges of the section are formed by meridian planes, spaced 1) degrees as indicated in Fig. 8, then the angular width, w, (=sagittal angle the apex of which lies at the focal point) of a section Whose center is at the angle a on the generating parabola is w=p sine a degrees (360) and the width of the reflected beam is W sine a+s degrees (361) The first apparent condition for maximum uniformity is then W1= W degrees (362) If the light source is spherical, and of width 8.; degrees as measured from the vertex of the paraboloid then at any angle a Equations (359) and (362) must give identlcal values I l,+s, cos =1) sine 0+8, cos degrees (363) and this equation is then the basis for finding the proportions of the various sectlons. To have the width W, equal at'the generating center angles, a, b, 0, etc. we have 1 s, cos degrees (364) a Micetc. and the corresponding widths of sections to give equal beam width W, are found from W =p sine a s, cos;

=1), sine b-l-s, cos; degrees (365) c =1). sme 0 +8, cos

To take a specific case let us assume that a beam of forty-five degrees width is desired, and that the spherical light source subtends an angle of fifteen degrees from the vertex of the generating paraboloid or so=15 deg.

The vertex of the paraboloid may be cov- V ered by a single mirror cut in a polygonal form so that its angular diameter is =45-15=30 deg.

and again we assume somevalue Z so as to i make the second correction in s for the varying angle formed by the source. 1

Take l 32 deg, then the center of the third zone is at 46 16=62 deg. and

45'=z.+15 cos 31 'z.' -4511=34 The inner edge of the fourth zone at' 46+ 34=80, deg. p

and the length of this zone may be estimated at 36 deg. and Equation (364) used again to determine l These successive approximations -may be made by the aid of Fig. 1 where the angle subtended by a spherical source of diameter 1 deg. as measured from the vertex is plotted to show the relation between 8 and the gencrating angle a. Thus, at 0 deg. the source in the above example contributes 15 deg. to the beam width, but at 90 deg. the added width is 7.5 deg. and at 120 deg. only 3.75 deg. With these corrections made all zones will give nearly the same width ofbeam. Two factors have been omitted in thus computing the beam width. These are the added width due to the corners of the sections extendin out farther than the ends of the two center lines, and the decreased width due to the application of the ten percent rule to the determination of the edges of the soft edged beam.

' These two factors are opposed and the approximation derived as above is substantially correct.

The width of the section of mirror is dependent upon the generating angle a, as

7 given in Equation (365) and therefore each zone should contain a different number of mirrors. It will be observed that the additional Width due to the size of the light source, as given in Equations (364) and (365), has an equal influence on the two dimensions of the sections, and therefore no influence at all upon their relative values in the same zone. Substituting the numerical values previously obtained for the central angles, a, b, 0, etc. the values of p p etc. are found to be 360 deg. 60 deg. 38 deg. and 40 deg. Dividing these widths into 360 deg.

the number of mirrors in the successive zones are found to be 1, 6, 9, and 9. The general rule is that a single mirror suifices in all cases to cover the apex of the paraboloid, and the number of mirrors reaches a maximum at 90 deg. and then decreases to a theoretical single mirror when the edge of the paraboloid is at infinity, that is, when the generating an gle approaches the hypothetical condition of equaling 180 deg.

TABLr; XXIL-Angular width and length of sections for greatest uniformity of beam (15 deg. source).

Zone From To Center TAmrfh Source Beam z g rorsin No. Deg: Deg cg. Deg. Deg. Deg. Deg zone Mir- In the construction of a mirror from the above data it will be seen that a practical thing to do is to use eight sections, or perhaps ten, in the" three outer zones. This would make the mirror easier to construct, and it would not reduce the number of active sections or the central beam intensity. The following tablegives some of the angles in the simplified design etc. and

TABLE XXIIL-Angular width of sections in simplifled design.

Eight mirrors per zone (me- Tan mirrors per zone (mezone ridian sagittal angles) radian sagittal angles) No.

Angles Eflect some Total Angles Eflect Source Total 360 so 15 45.0" 360 so 15.0 45.o 45 23 14.4 37.4" 36 18 14.4 324 45 40 11.0 5l.0 36 32 11.0 43.0" 45 44' 6.2" 50.2 36 35 6.2 41.2"

There are here three designs to choose from, and the choice will be influenced by the beam formation, which includes such factors as uniformity of intensity', -width and central intensity. These factors can best be evaluated after the beam characteristic has been determined, as will appear.

(2) Disk urce.-The proportions of the sections in a mirror to be used with an are, or disk source, are influenced by the fact that from the outer parts of the mirror the source seems to be elliptical in outline. In making allowance for the width added to the beam by the dimensions of theme (or monoplane fila- =l +s cos 2 cos b degrees 7 (366) c =l,+s, cos 5 cos c a W,,=p., sine a-i-s, cos 5 =1) sine b+s cos; degrees (367) =1), sine c +s, cos g etc.

In the Equation (366) above, the factors cos a, cos 6 etc., are the factors for a disk-like source of light; other forms of light source have other factors and therefore 7 may be substituted for cos a, cos 1), etc. in a generalEquation for any light source.

' 'We have seen in Equation (362) that W,,=W then substituting 7" for cos a in (366) above, that a 1).. sm a+s cos 1,, s cos% (1') degrees,

and

p sin (1 =1 s,, cos s. cos g degrees,

and a i I l -l-s cos g (f-Z) sin adegrees (367-a) This final Equation (367'-a) is then the basic equation for a reflector according to the present invention.

It should be noted that the size of the source plays a part in the relative dimensions of the sections because the factors of s are not the same in Equations (366) and (367) and in this respect the design will differ from that for a spherical source. I

I Uhord or tangent? In the sectional view in Fig. 3 the tangen-' tial mirrors are seen to leave open joints at the ends of'the sections. This is not a desirable form of construction and if an attempt is made to bringthe ends together the optical adjustment of the zones will be destroyed. It

is therefore necessary to find some other method of construction. The basic point in determining the size and position of each section isits point of tangency to the generating paraboloid. If, however, we change our method of approach and assume the junction points to be on the paraboloid, then there I is no point of tangency and the mathematical relations are altered. This construction on the basis of chords is much to be preferred and is one often used in practice.

The equation for determining the angular widthand length of the sections in order to secure uniformity remain unaltered when dealing with chords in place of tangents, but as will be seen later, :a slightly different interpretation must be given to the angles and a different value is found for the focal length.

Use 07 chords.

Mirror sections of 30 deg. lengths and bounded by meridian planes spaced 30 deg.

are shown in 4 along with the generating paraboloid. iOnly the corners of the s'ections touch the paraboloid and the ends of the sections are chords or circles, while the edges are chordsoftho paraboloid. The optical properties rof the center line of each section are of prime importance and the adjustment of section and lightsource r'nustbe made from the data of the center line.

' The height, or 3/ value of a point on the we find from (366) and (367 center of the end of a section of idth deg.-is 5 V asillustrated in Fig. 5', and if all zones contain the same numberoof mirrorsthen p is a common angle for all, and the relation between 3 and'y is constant for all parts of the paraboloid. Placingthis value. of y, n

the equation of the parabola,'weget "=4 as s at? i 9) 1 y iFac cos? and th1s is the equation of a new parabola, initegeriels.

that passes through the junction points'of the various center lines. The focal length of the derived parabola is F Cra g I (370) and the light source should therefore be moved toward thevertex accordingly. The generating and derived parabolas have a common vertex and the origin for measurlng F and F isa common origin.

If a chord in a meridian plane is investi-- gated for reflection of light originating at the focus it will be found that the axis of the reflected beam is obviously not that best] adapted for giving the best concentrationpf light, and the equation derived belowgivesi a I further shift ofithe focal point to bring one particular mirror zone. into optical adjustment... v 1 p In Fig. Gtheparabola is the one derived to pass through theends of the meridian center lines, and tl'lQSpCtiOn extends from the vertex to a point (atat an angle 1 from the ax1s.-.

.Then it I w w .=t8 (371) i a,

betheangle between thechord from (O, 0) to and the tangent at vertex,

t... 11;?! eal" I a 2 2 i 7 FEE-@972. (3 4 I Let the lig it sbiircj a atite at aapoia s a distance F; fromthevertex(Fig. 6)

" consider so that the entire chord subtends an angle 26.

.When this condition is fulfilled the ra s reflected at the (0,0) point will have-a ownward slope equal to the upward slope of the ray reflected at (w y The downward slope is seen to be fixed and equal to b degrees because the ray striking at (0,0) does so at zero angle is deflected by twice the'angle between the mirrors and the'plane at the vertex, that is, twice 6.

-If this value for the new focal length "is, substituted in the expression for the generating parabola we get F .=cos* 1 mugmag. .)F 2377 for the correctfocal point for thegfirst zone of mirrors. 1 It will be noticed that the uation for F, was derived for a zone exten in from the axis. The equation cannot, there ore, beexpected to hold accuratelyffor all zones, but

a number of trials have shown it to be as correct as need be for the design of this type of mirror, where 1 deg. plus or minus is not of great importance. 5

In Fig. 7 .several curves are given for the focal len h F .when I and p vary from zero to forty egrees. A mirror section 30 deg. by 30- deg. reduces the focal length 91.5 per cent of the focal length of the'generating parabola, while sections 40 deg. by 40 deg. reduce it 'to p)er cent. ,These reductions are of I 1e importance because it is custom-' ary tomake the focal length as small as will permit the use of some predetermined lamp bulb, and unless proper account is taken of the difiere'ncebetween F and F it will not be possible to bring-the lamptothe focal point of the, finished reflector. c.

In case the .severalmirror zones contain I different numbers of sections it wouldv seem the wise thing to take the combination of p I and Z that 'ves the smallest value of F and thus guar against the lamp bulb coming against the mirror when focused. An additional reason is the loss of focusing s ace,

due to the, clips or other fasteners that old the mirrors place. I y

If the light'source is shifted to the new focal length F all theangular relations will bealtered in some degree. The pur ose ofthe'change is to make the upward an downs ward reflectionequal for the top and bottom edges of same section, but additions to the upward angle will not be exactly balanced by the lessening of the downward angle and the total width of the beam will be slightly altered. This alteration is in the nature of a second order correction, (that for the size of the light source being of the first order) and is not of suflicient size to warrant oing through the design a second time in or er to compensate for it.

By referring to Figures 11 and 12it will be seen how the reflector of the invention is that the transverse angular dimensions of the anels in the third zone are different from simllar dimensions in the panels of the second zone. There is a special case, however, in which the number of panels in two adjacent zones is the same as indicated in Fig. 13. This is the case when sin b of Equation 367 is equal to sin c of the same equation. This only occurs, however, when, for example, c exceeds 90 degrees by the same amount that 6 falls short of 90 degrees.

I If in Fig. 13 an axial plane 1414 is passed through the centers ofthe panels radially disposed along the different zones, as indicated in Fig. 13, the plane will cut the panels along lines which in Fig. 14 are shown as chords. Now the ends of these chords may be used to determine a second or derivative paraboloid shown in Fig. 14 with a second or derived focal distance F,. This. new focal distance is the focal distance F of Fig. 6

from which the operating focal'length F (Fig. 6) is obtained.

What I claim as new and desire to secure by Letters Patent of the United States, is,

1. A reflector composed of flat panels assembled about an axis and having a general contour of a focalizing surface, each panel having angular dimensions measured about the focal point of the focalizing surface related as follows:

where Y l.=angle subtended in the longitudinal plane by longitudinal center line of panel,

1 P.=angle between meridian planes forming sides of panels whose transverse center lines are at an angle a with the axis of the reflector,

s angle subtended in the transverse plane by the light source, the point of measurement being the intersection of the two center lines of the panel,

a=angle between axis of complete reflector and the intersection of the two center lines of the panel,

f=a factor to compensate for foreshortening of the source of light.

2. A projector comprising in combination with a light source a plurality of reflecting elements assembled about the axis of a focalizing surface for producing a light beam, each reflecting element being polygonal and having a lurality of its corners in the focalizing sur ace, each of said elements having angular length and width equal to the angular length and. width respectively of a predetermined distant area along and normal to the axis of the projector to be illuminated by said beam less the angular width of the light source, the angular width of the light source being measured about the central point of the element, the angular length and width of said elements being measured from the focal point of the focalizing surface.

3. A rojector comprising in combination with a ight source a plurality of reflecting elements assembled about the axisof a focalizing surface for producing a light beam, each element being polygonal and having a plurality of its corners in the focalizing surface, each of said sections having angular length and width equal to the angular length and width respectively of a predetermined distant area along and normal to the axis of the projector to be illuminated b said beam less the angular width of the light source, the angular width of the light source being measured about the central point of the element, the angular length and width of said elements being measured from the focal oint of the focalizing surface, each of sai elements being flat.

4. A reflector formed by a plurality of flat sections, a lurality of flat reflector panels adjusted to e in internal contact with a focalizing curve, the length and width of said sections being so chosen that the divergence of the rays originating in the focal region of the focalizing curve is substantially equal for all sections, said panels being also arranged about the axis of the focalizing surface each panel having angular length l and I angular width p related as follows:

p =angle between meridian planes form,

(degrees) ing sides of panels whose transverse center length and width lines are at an angle a with the axis of the refiector,

s =angle subtended in the transverse plane by the light source, the point of measurement being the intersection of the two center lines of the panel,

a=angle between axis of complete reflector and the intersection of the two center lines of the panel,

f=a factor to compensate for foreshortening of the light source (ordinarily f=cos a for a disk-like source of light).

5. A reflector comprising in combination with a light source, a plurality of reflecting elements assembled about an axis in the form of a focalizing unit about said axis, each reflecting element having an angular length 1, and an angular width P equal to the angular respectively, about the focal point of the unit, of a predetermined area to be illuminated less the angular width of the light source, the angular width of the light source being measured about the een-.

tral point of the element, the angular length l and the angular width P being so related that Z +S f cos %=P sin a+S cos 3-- 6. A projector comprising in combination with a light source, a plurality of reflecting elements assembled about the axis of a focalizing surface for producing a light beam, each section being polygonal and having a plurality of its corners in the focalizing surface, each of said sections having angular length 1,, and angularwidth P equal to the angular length and width respectively of a predetermined distant area along and normal to the axis of the projector to be illuminated by said beam less the angular width of the light source, the angular width of the light source being measured about the central point of the element, the angular length and width of said elements being measured from the f0- cal point of the focalizing surface, the angular length Z and the angular width P being so related that Z +Saf 00822 2 P sin a+S cos FRANK A. BENFORD.

CERTIFICATE OF CORRECTION.

Patent No. 1,726,379;

FRANK A.

Granted August 27, 1929, to

BENFORD.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 1, line 90, for the word "radially" read "radically"; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 15th day of October, A. D. 1929.

(Seal) M. J. Moore, Acting Commissioner of Patents. 

